The present invention relates generally to power and control systems and more specifically to power and control systems used to control the temperature of a workpiece such as a semiconductor wafer and/or to control the temperature of the workpiece chuck on which the workpiece is held.
In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC process steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.
Typically, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For example, circuit probe testing is increasingly performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held stationary relative to a vacuum support surface of a prober machine which electrically tests the circuits on the wafer. The prober includes a group of electrical probes which, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits"" responses to the excitations.
In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure for the chuck is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.
The chuck can also include a temperature control system which raises and lowers the temperature of the chuck surface and the wafer as required to perform the desired temperature screening of the wafer. It is important to the accuracy of such testing that the temperature of the wafer and, therefore, the temperature of the chuck surface, be controlled as accurately and precisely as possible.
Various approaches to controlling the wafer temperature have been employed. In one prior system, the chuck includes a circulation system through which a cooling fluid is circulated. The cooling fluid is maintained at a constant cold temperature and is circulated through the chuck. Temperature control is realized by activating a heater which is also located in the chuck. The heater is cycled on and off as required to heat the chuck and the workpiece to the required temperature.
In another prior system, both a temperature-controlled fluid and a chuck heater are used to control the workpiece temperature. In this system, the fluid is used to bring the workpiece to within a certain tolerance of the desired set point temperature. The heater is then cycled as required to trim the temperature to the set point.
Temperature control systems can typically include heat exchangers such as condensers and evaporators for heating and cooling a medium such as circulated air or liquid. For example, in an air conditioning system, a refrigerant such as freon is circulated through the system to remove heat from circulated air. Specifically, the evaporator heat exchanger is in close proximity to the air to be cooled. As the freon evaporates, it removes heat from the air to cool the air.
In one type of conventional evaporator, the cold refrigerant enters the evaporator through an orifice and is allowed to flow over metal plates arranged parallel to each other with spaces between them for the refrigerant. As the refrigerant flows between the plates, the plates cool. In one configuration, the cooled medium, e.g., air, is routed through the evaporator such that it also flows between the plates on the sides opposite the sides in contact with the refrigerant, and the air is cooled by the cold plates.
In many settings, it is important that the evaporator be extremely efficient and provide as much cooling as possible. For example, in a workpiece chuck in which a circulating fluid is used in controlling workpiece and/or chuck temperature, it is important that the workpiece and/or chuck be heated and cooled very quickly and with a high degree of accuracy. In such a case, the evaporator should have very high efficiency. The efficiency of the evaporator can be affected by many factors. For example, in conventional evaporators, the low-temperature refrigerant enters the evaporator through the orifice at high pressure. It is in general distributed over the plates in an uncontrolled and uneven fashion, which results in a loss in efficiency. It would be beneficial to have the refrigerant distributed evenly over the plates such that the plates are cooled evenly. This would provide even cooling of the circulating temperature controlling fluid and also provide the most cooling for a given set of system parameters.
In accordance with the invention, there is provided a highly efficient heat exchanger and temperature controlling method which overcome the drawbacks of the prior art. The heat exchanger of the invention includes an input port through which a refrigerant enters the heat exchanger as well as a plurality of plates which receive the refrigerant. A tilted deflection surface is located near the input port and is contacted by the refrigerant as it enters the heat exchanger. The tilted deflection surface directs the refrigerant such that the refrigerant is distributed over the plates. In one embodiment, the heat exchanger of the invention is an evaporator.
In one embodiment, the deflection surface is formed as part of an insert which is mounted inside the input port through which the refrigerant enters the heat exchanger. The input port can include an orifice though which the refrigerant enters the heat exchanger. In one embodiment, the orifice is formed as part of the insert mounted in the input port. The tilt angle of the deflection surface is selected such that the refrigerant is evenly distributed over the plates, resulting in uniform efficient cooling in the evaporator.
The heat exchanger of the invention is mounted such that the cold refrigerant enters the heat exchanger at the top of the device. The refrigerant flows over the plates to the bottom of the device where it exits the device through an output. In many settings, the refrigerant flowing through the evaporator is accompanied by a lubricant used by other devices in the refrigeration cycle, such as a compressor. Because the refrigerant and lubricant flow from the top to the bottom of the evaporator, the lubricant simply flows out of the device by gravity and need not be recovered by such means as a capillary tube, as used in prior devices.
In one embodiment, the heat exchanger, e.g., evaporator, of the invention is used in a temperature control system which operates to control the temperature of a circulating fluid. For example, in one embodiment, the fluid is circulated through a workpiece chuck in which the temperature of a workpiece, such as a semiconductor wafer, can be controlled. The temperature of the fluid is controlled by the temperature control system that uses the heat exchanger of the invention. In such a system, the heat exchanger of the invention includes a fluid input and a fluid output such that the temperature control fluid can be circulated through the device. The fluid runs through the device through the plates which are cooled by the refrigerant. Heat is therefore removed from the fluid and transferred to the refrigerant via the plates. In one particular embodiment, the circulating temperature control fluid comprises methyl nonafluoroisobutyl ether. In an alternative embodiment, the fluid comprises methyl nonafluorobutyl ether.
The heat exchanger of the present invention can be used in many temperature control settings. In particular, the heat exchanger of the invention can be the evaporator in a fluid temperature control system used to control temperature in a workpiece chuck by controlling the temperature of a fluid circulated through the chuck. Such a system is described in, for example, copending U.S. patent application Ser. No. 09/001,887, filed on Dec. 31, 1997, of the same assignee as the assignee of the present application. The contents of that application are incorporated herein by reference.
The present invention provides numerous advantages over conventional heat exchangers, in particular, evaporators. The tilted deflection surface is used to evenly distribute the refrigerant over the evaporator plates. As a result, more uniform cooling of the plates is achieved. This results in more efficient heat transfer from the circulating temperature control fluid and more efficient overall operation of the host temperature control system.